This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved optical system adapted to produce continuous tone light images having substantially uniform intensity.
A typical electrophotographic printing machine has an optical system which forms a light image of an original document. The light image irradiates the charged photoconductive surface to record an electrostatic latent image thereon. The electrostatic latent image is then developed with charged particles to produce a powder image thereof. Thereafter, the powder image is transferred to a sheet of support material and permanently affixed thereto forming a copy of the original document. The foregoing process is described, in detail, in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.
Multi-color electrophotographic printing is substantially similar to the heretofore discussed process. However, rather than forming a total light image of the original document, successive filtered light images are formed to produce successive single color light images which are partial light images of the original document. These successive single color light images irradiate the charged photoconductive surface to form successive single color electrostatic latent images thereon. Each single color electrostatic latent image is developed with toner particles of a color complementary to the respective filtered light image. The single color toner powder images are transferred from the electrostatic latent image to a sheet of support material, in superimposed registration with one another. After the multi-layered toner powder image is formed on the sheet of support material, it is permanently affixed thereto forming a color copy thereon.
Conventional screening methods may be employed in an electrophotographic printing machine. Such methods produce the effect of tone gradations plus variations in dot size. In the highlights, the dots will be small and will increase in size through the intermediate shades until they merge together in the shadow region. At the extremes, there will be complete whiteness at the highlight end of the tone scale, and nearly solid black at the shadow end. This type of tone structure can be reproduced in an electrophotographic printing machine.
Numerous techniques for screening have hereinbefore been developed. By way of example, U.S. Pat. No. 2,598,732 describes the use of a screen in electrophotographic printing. Another example is found in U.S. Pat. No. 3,724,944 which relates to the use of a screen in a belt configuration.
In addition to forming a continuous tone light image of an original document, it is highly desirable to insure that the light image irradiating the charged photoconductive surface has substantially uniform intensity. The density of toner particles deposited on an electrostatic latent image is dependent upon the voltage level of the development system relative to the electrostatic latent image. Toner particles are attracted to those surfaces of the photoconductive surface having a voltage thereon greater than that of the development system. The areas of the photoconductive surface which are irradiated by the single color light image are discharged. The degree of discharge is dependent upon the intensity of the light rays impinging thereon. However, it is well known that the illumination at an image point is in proportion to the Cos.sup.4 of the solid angle between the illumination point and the image point. Thus, it can be seen that the illumination of the photoconductive surface will fall off quite rapidly as the solid angle increases. Various approaches have been devised to compensate for this effect. Typically, a sheet of opaque material having a "butterfly" slit formed therein is employed. The area of the butterfly slit is inversely proportional to the illumination profile. Other techniques employ masks having slits of this type integral with the lens system. Techniques of this nature are examplified by U.S. Pat. No. 3,224,327 and U.S. Pat. No. 3,469,916 which relate to a mask in the path of a light image having an opening in the middle narrower than at the end.
However, it is highly desirable to have a capability of varying the intensity of the light image transmitted to the photoconductive surface. The foregoing will compensate for aging effects of the light source and permit the selection of the appropriate slit for the functional or pictorial mode of operation. In addition, during the calibration process of the electrophotographic printing machine, the appropriate size exposure slit is chosen. This is achieved by choosing different exposure slits of varying widths. Heretofore, numerous exposure slits have been contained loosely in the electrophotographic printing machine enabling the operator to select the appropriate slit during calibration. Frequently, this results in the loss of the exposure slit. The foregoing increases the calibration time and cost of parts.
Furthermore, in multi-color electrophotographic printing machine, it may be highly desirable to employ a screen in certain modes of operation in conjunction with the exposure slit (pictorial mode) and, in other modes of operation, to space the screen from the optical light path (functional mode). Thus, in a photographic or pictorial mode of operation a screen would be employed. Contrawise, if the machine is operated in a functional mode wherein graphs or line copies are being reproduced in a multi-color mode, a screen is not desirable. In addition, various types of screens may be employed in order to provide an appropriate composition for the type of copy being produced.
Accordingly, it is a primary object of the present invention to provide an electrophotographic printing machine having an improved optical system arranged to produce continuous tone light images of variable light intensity.